Just in case you thought the UltraFine Display being limited to 4K resolution when connected to a USB-C device like the MacBook or iPad Pro, as opposed to showing 5K resolution when connected over Thunderbolt 3, this sort of problem is not new:
When you connect an Apple Cinema Display (30-inch DVI) to your computer, you may find that the highest available resolution is 1280 x 800 even though the optimum resolution for the display is 2560 x 1600. This can happen if the display has a single-link DVI connection instead of a dual-link DVI connection.